The present invention is directed to a method for recovering fluorine and phosphate values from waste waters or pond waters resulting from the production of wet process phosphoric acid. More particularly, the present invention is directed to a method of recovering calcium fluoride and calcium orthophosphate from phosphate waste waters. Moreover, the present invention provides a method of neutralizing the waste waters so they can be discharged into rivers and streams without fear of pollution or recycled as process waters in the production of phosphoric acid. For purposes of this invention, phosphate waste waters and pond waters shall be synonymous terms for waste.
In a wet process phosphoric acid plant using the dihydrate technique, about 50 gpm per ton of product P.sub.2 O.sub.5 per day of waste water is directed to a waste pond. The pond water contains from about 0.1 to about 5% fluorine, from about 0.1 to about 5% P.sub.2 O.sub.5, from about 0.1 to about 2.5% SiO.sub.2, from about 0.1 to about 0.5% dissolved calcium and from about 0.1 to about 0.5% soluble sulfate salts. The fluorine in pond waters is a major concern because it can present an ecological hazard. The loss of P.sub.2 O.sub.5 values in pond waters is considered a serious problem not only because it is a valuable product but also because it can present an ecological hazard.
Traditionally pond waters have been passed through settling basins prior to their discharge into rivers and streams. At times, pond waters are treated with limestone and lime to precipitate out fluorine and other values before discharge to meet pollution control laws and regulations. In the settling basins, the various chemical values, such as fluorine, P.sub.2 O.sub.5, calcium, etc., decrease so that the pond water when discharged into the streams contains lesser, but appreciable, amounts of these materials. Not only do discharged pond waters add chemical values to streams, but they also cause a decrease in the pH of the streams. Pond water typically is acidic and has a pH from about 1 to about 3.
Workers in the art have recognized the economic loss and ecological problem of pond waters and have developed methods of treating pond waters. However, it appears that none of these methods have been economically attractive or feasible since none of the methods are in commercial use in the United States. For example, D. R. Randolph developed a method which is disclosed in U.S. Pat. No. 3,625,648. The Randolph method comprises treating pond water with milk of lime to adjust the pH of the resulting slurry to between about 3.2 and 3.5 whereby 99% of the available fluorine is precipitated out as calcium fluoride. The calcium fluoride is separated from the aqueous phase and treated with sulfuric acid, or other strong acid, to liberate the hydrogen fluoride gas and yield a slurry of gypsum, sulfuric acid and phosphoric acid. The latter slurry can be recycled back into a conventional wet acid phosphoric acid process to recover the P.sub.2 O.sub.5 values. The HF gas can be upgraded pursuant to conventional methods. The aqueous phase, after removal of the calcium fluoride, is treated with an additional 10% milk of lime to adjust the pH to between 4.7 and 5 to precipitate out dicalcium phosphate. Dicalcium phosphate is separated from the aqueous phase and is upgraded in a conventional dicalcium phosphate plant or cycled to a conventional wet acid phosphoric acid plant to recover the phosphate values. The aqueous slurry is then treated with additional milk of lime to adjust the pH between 6 and 7 wherein further solids precipitate out. The solids are separated from the almost neutral aqueous phase and passed to waste. The aqueous phase is then recycled as process water to the phosphoric acid plant or dicharged into streams or rivers.